Semiconductor photolithography generally involves a sequence of processes in which a photoresist layer is applied to, e.g., a semiconductor wafer, and the photoresist layer is exposed to radiation in a pattern corresponding to a desired semiconductor processing pattern. The exposed photoresist is typically then processed to form a patterned barrier film for a subsequent wafer process or processes. Photoresist films generally and historically consist of a polymer resin, in addition to other optional components. A polymer-based photoresist film can be processed with radiation to induce photochemical reactions in localized regions of the film corresponding to a pattern of the radiation, and these selective reactions enable a precise optical-based mechanism for producing a desired barrier pattern in a photoresist film.
In a conventional photolithographic process, a polymer photoresist is applied to a wafer as a liquid by spin-coating one or more layers of the photoresist on the wafer. This is accomplished by, for example, placing a few drops of a liquid polymer-solvent solution on a wafer as the wafer is rapidly spun on a support platform. The spin-applied liquid polymer is then typically heat-treated to evaporate the solvent and stabilize the film. Next, the film is exposed to radiation through a patterned mask, e.g., a glass plate having chrome patterns, such that only selected regions of the film are exposed to the radiation and undergo radiation-induced chemical reactions. Typically, a so-called positive-tone photoresist film is developed after this exposure step in an aqueous or solvent liquid developing solution to remove the film from the exposed areas; a so-called negative-tone photoresist film is developed to remove the unexposed areas of the film. After development, both positive- and negative-tone photoresist films result in an acid-resistant, patterned coating over selected regions of the substrate.
The optical resolution limits of this conventional liquid-developed photolithographic process have led to the development of many alternative and advanced photolithographic techniques and photoresist films. The class of photoresist films known as silylation resists provide one approach for achieving fine lithographic line widths by employing a dry, rather than liquid-based, photoresist development system. In a typical silylation-based lithography process, a liquid silylation photoresist is spin-applied to a wafer in much the same way a more conventional liquid polymer resist is spin-applied. In a positive-tone silylation resist film, exposure to a pattern of radiation induces a change in the radiated polymer regions that results in a surface permeation barrier, the surface barrier suppressing permeation of a silicon-containing compound into those regions in a later processing step. A negative-tone silylation resist film is characterized by photo-induced chemical reaction of the film in unexposed regions of the film, those unexposed regions providing a selective permeation barrier to a silicon-containing compound in a later processing step.
Considering the case of a positive-tone silylation photoresist, the silylation resist consists of a polymer resin that includes hydroxyl bond groups dispersed throughout the resin. The photochemical changes induced by the radiation exposure of the film results in the illuminated regions being crosslinked. Subsequent exposure of the film to a so-called silylation reagent, in the form of a silicon-based liquid or silicon-containing vapor, results in a chemical reaction between the silicon-containing reagent and the hydroxyl groups in those regions of the film that were not illuminated. The silylation reagent does not penetrate into the illuminated, crosslinked areas of the film. A desired pattern corresponding to the radiation exposure pattern is then produced by exposing the silylated film to an appropriate dry-development process, e.g., an anisotropic oxygen plasma etch. The film areas that were not crosslinked and that were therefore silylated convert to an SiO.sub.x film in the oxygen plasma and form a surface etch barrier to the plasma. The cross-linked areas are, however, attacked by the plasma and are removed. This anisotropic etch step is analogous to the liquid development step of conventional photolithography and results in patterning of selected photoresist film regions to produce a patterned barrier film for subsequent wafer processing.
Spin-applied liquid silylation resists have historically been limited by the homogeneous nature of the constituent species in the starting resist liquid. The volume concentration and localized profile of hydroxyl sites in a silylation resist film determine to a degree the contrast, resolution, etch resistivity, and other such characteristics of the film. For many silylation photoresist applications, it would be desirable to tailor the profile of constituent species in a photoresist film based on performance goals for specific lithographic or etch processes. But profile tailoring of spin-applied films is in general very limited, if not impossible, for several reasons. First, the chemistry of spin-applied layers is limited to the original polymer synthesis in the starting liquid. In addition, spin-applying of thin films with the uniformity required for sub-0.25 .mu.m photolithographic resolution is very difficult to physically control, particularly over wafer topography. Furthermore, the "stacking" of multiple spin-applied film layers introduces an abrupt physical and chemical interface between each of the film layers.
Multi-layer resists (MLR) have also been investigated as an alternative to conventional wet-developed, single layer resists. Examples of MLRs include combinations of spin-applied and dry-deposited films. These MLR systems are not, in general, based on a silylation patterning process and are thus fundamentally limited because they cannot enable a silylation mechanism, and are further limited by the use of more than one development step. In addition, MLRs inherently have at least one abrupt interface that is a chemical, physical, or both a chemical and physical interface.
Spin-applied silylation photoresists, and spin-applied polymer photoresists in general, all pose costly waste and disposal problems that have chronically faced the semiconductor fabrication industry. In addition, concerns for operator safety during handling of spin-applied photoresists have historically been a processing issue. Aside from safety considerations, spin-applied photoresists pose fabrication limitations that are of increasing importance as the complexity of fabrication development advances. For example, a selected fabrication substrate or film may be reactive with the solvents in a liquid photoresist solution, or may be reactive with liquid developers used in a photolithography process to develop an exposed photoresist film. Beyond the limitations of liquid photolithography chemicals themselves, a selected fabrication substrate or film may be easily contaminated by handling during spin-coating of a photoresist film on the substrate, and may be prone to unwanted oxidation in ambient exposure during the spin-coating process. As a result, spin-applied liquid photoresists are sub-optimal for many substrate materials.
A more complicated limitation is posed by the various substrate materials that currently are not available in standard sizes and shapes, making such substrates entirely incompatible with a standard spin-coating process. For example, exotic substrates, such as HgCdTe substrates, are typically supplied in rectangular or other nonsymmetrical, rather than circular, geometry, thus making uniform spin-coating of photoresist films not possible as a practical matter for production processes. Similarly, the substrates required for many fabrication applications are of such large dimensions that physical spinning of the substrates is not possible without specialized equipment. For example, very large substrates employed in fabricating flat panel display systems cannot be physically spun. As a result, complicated and expensive alternative thin film application processes, such as squeegee techniques, have been developed to produce photoresist films on such large substrates. The topological dimensions of a substrate also limit its ability to enable a uniform spin-coating of photoresist. Many substrates that support fabrication of mechanical components as well as electrical components, as is the case for, e.g., microelectromechanical sensor or actuator systems, include varying degrees of topography that cannot be effectively coated with a spin-applied photoresist film. Thus, while spin-applied photoresist films have been sufficiently developed to enable photolithography on very standard substrate materials and sizes, such photoresist films are inadequate for and/or incompatible with an increasing number of substrates encountered as semiconductor fabrication processes continue to evolve.